[Emissary of Death] wasn't a skill that should exist. Well, at least that's what Astaroth said as soon as the prompt popped up. According to him, the Precursors had a set of rules called Primiosis Laws.
One of the most significant laws was never to resurrect oneself or others, and the Precursors had ensured none of the skills related to resurrection ever saw the day of light. That's why it was most surprising when Ashton received a reality-shattering ability like [Emissary of Death].
However, Ashton had a rough idea of how and why he got the ability. His father was a Precursor who had seen his future and helped him seal the ill effects of Grim Reaper's scythe.
Although it was only a theory, his father might have imparted the skill to him while they were saying their final goodbyes to each other. It had to be that because Ashton had no other reason to why he would receive such a skill.
